Solid-state sensors may be used in a wide variety of applications. For example, chemical solid-state sensors may be used for real-time analysis of chemical mixtures in both continuous and discrete sampling modes. Similarly, biological sensors can be used to detect biological agents and hazards and radiation sensors can be used to detect types and amounts of radiation.
The sensors may be used to detect a single component in a complex mixture, such as a toxic molecule in ambient atmosphere, analyze multiple components in a composition, or perform characterization and quality assessment of complex mixtures—e.g., as used to characterize odors, tastes, smells, etc., by pattern recognition methods using array-based sensors.
Typical solid-state sensors generally include a detection or receptor element and signal transduction means. The receptor layer interacts with the target specie(s)—e.g., by physical absorption or physisorption, chemisorption, microencapsulation, or the like. The transducer converts a change at the receptor surface into a measurable electrical signal. The signal transduction, or coupling of signal between the receptor and the transducer may be linear, nonlinear, logarithmic or exponential in relation. The coupling relation between the two elements generally determines the sensitivity of the device.
A variety of signal transducer elements, such as potentiometric sensors, amperometric sensors, conductometric sensors, field effect transistor (FET) based sensors, optical sensors, thermal sensors, gravimetric or piezo-electric sensors, and the like, have been developed. FET devices may be particularly desirable because the FET devices exhibit relatively fast and sensitive signal transduction, are relatively easy to use, and are relatively easy to integrate with other sensor components.
In the case of FET devices, the metal gate of the field effect transistor device is either replaced or coated with a sensitive thin film, insulator or membrane, which acts as the signal detection element. The FET devices work on the general principal of detecting shifts in localized electric potential due to interactions at the device surface. The FET device transduces a detection event into an electrical signal by way of change in the conductance of the channel region leading to a change in the drain current. The FET device may be operated as a sensor either by biasing the device with constant gate voltage and measuring the change in the current or by detecting the change in gate voltage required to maintain a constant current.
Metal-Oxide-Semiconductor FET (MOSFET) type sensors are often operated in inversion mode, where inversion current is established in the semiconductor channel by biasing the metal gate of the MOSFET. In these devices, target molecule binding at the sensitive thin film or a change in radiation level modulates the minority charge carrier density in the inversion channel. Hence, inversion current in a bulk p-type MOSFET decreases upon addition of negative charge to the device surface.
Although such devices and transducer elements have been shown to work for some sensing applications, the non-FET devices are relatively bulky and expensive, and the FET-based devices may be relatively unstable and exhibit relatively low sensitivity. Accordingly, improved sensors and methods of making and using sensors are desired.